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Author Topic: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe  (Read 80070 times)

AbruptSLR

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The PIG/Thwaites Glacial drainage basin system is the soft underbelly to the potential collapse of the WAIS this century, and the destabilization mechanisms for initial time period from 2012 to the 2040 to 2060 time frame are the most critical aspects of the entire WAIS collapse hazard analysis scenario.  This tread thus focuses on this topic, while later threads will focus on the 2050/2060 to 2100 time period, for the Amundsen Sea Embayment, ASE, the Ross Sea Embayment, RSE, the Weddell Sea Embayment, WSE and the Bellingshausen Sea areas.  I will call the timeframe from 2012 to 2040/2060 the collapse initation period and the timeframe from 2050/2060 to 2100 the collapse main period.  For this collapse initiation period scenario for the PIG/Thwaites system, the Thwaites Glacial drainage system is particularly unique and critical and will be the focus of this hazard analysis; however the PIG collapse initiation period started earlier (and has been more extensively modeled) than for the Thwaites Glacier, therefore the PIG response will serve as a base case that will be adjusted to project the expected response for Thwaites.
The PIG grounding line reached the crest of a submerged ridge leading to a negative slope by about 1994 and that grounding line retreated about 25km from 1994 to 2010 (a 16-yr period) and the width of the PIG gateway is about 40 km.  I believe that by 2012 a subglacial cavity (and intercepted lake) for the Thwaites Glacier, TG, has extended about 55 km (from the 1994 grounding line) to a submerged ridge leading to a negative slope (leading to BSB) with about the same gradient as for the PIG negative slope (see the first figure).  Now the collapse initiation of the TG is highly dependent on the vertical advective process (which will be shown in the second post) in order to extend this pre-existing 55-km long by 25-km wide by 0.8-km tall cavity into a cavity that is approximately 155-km long by 50-km wide by 1 km tall by about 2040 (as shown in the second figure), where it is expected to intercept another subglacial lake behind a damming ridge identified by Scheoder et al 2013 (see the "surge' thread).  As the Thwaites Gateway is about 50-km wide it is assumed that advection will widen it naturally by 2040.  In regard to lengthening the TG cavity by 100-km in 2040-2012 = 28 year, we make the following three adjustments to the 25 km that the PIG cavity extended over 16-years (see the third figure): (a) the factor for time 28/16 = 1.75; (b) for an expected increase in the average CDW temperature from 1.2 C to 1.8 C (a figure for a sensitivity analysis for ice volume loss with a cavity like that for TG with temperatue from Goldberg et al will be included in the next post) give a factor of: 1.5; and (c) the factor due to both the influence of the significant basal melt water system at Thwaites and the influence of the steeper ice surface slope: 1.5.  This gives 25 x 1.75 x1.5 x1.5 + 55 = 155-km long cavity.  Inaddition to presenting the Goldberg sensitivity information the next post will look at the PIG extension.
« Last Edit: February 28, 2013, 04:11:45 AM by AbruptSLR »
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #1 on: February 28, 2013, 03:24:28 PM »
Continuing from the last post, below I re-post a figure from Gladstone et al 2012 from the "surge" thread because it is such a key figure in that Gladstone et al ran over 1,000 computer runs with different assumptions for such key parameters as: basal friction, bathymetry, CDW temperature (note that the figure shows the SRES A1B case which the post in the "forcing" thread shows that we are currently following, & which has slightly more forcing than RCP 8.5 until about 2032), etc (and they bounded the reasonable runs between the heavy black lines) and they found the histogram of blue rectangles indicating the frequency count of the location of the end of the cavity for the runs made.  Gladstone et al also identified an area of local bottom bathymetry indicated by the green histogram showing that an area of possible local stability/pinning that  could temporarily stall the extension of the subglacial cavity after it extends another 40km from the 2010 location of the grounding line by about 2040, which is verified by Goldberg et al 2012 analysis (plan view instead of profile) shown in the second figure in this post (again reposted from the "surge" thread).  This matches the extent assumed for PIG by 2040 in the second figure of the previous post, while the activation of the side spur to the main PIG trough shown in the previous post assumes that the front of the PIG ice shelf  retreats past this side spur location by about 2020 to 2025 so as to active this side trough.
While in the previous post I used the historical (1996-2010) grounding line retreat for PIG as the basis for the future (2012-2040) groundling line retreat for Thwaites  adjust for time by 1.75, by temperature by 1.5, and by basal melt water interaction by 1.5, factors.  The 1.5 temperature factor is justified by the ratio of the model projected ice mass loss within a cavity similar to that for Thwaites for the 1.2 C to the 1.8 C curves shown in the third accompanyin figure (which I believe is related to the geometry of the Thwaites cavity immediately before 2012 after which the cavity intercepted the lake infront of submerged sea mount thus extending part grounding line at the tip of the cavity around to the eastern ice flow stream (see Tinto and Bell 2011 plan view) on the east side of the mount and allowing the grounding line on the west side of the mount [where most of the basal water channel flows] to begin to descend down into the BSB, this mount is assumed to be the primary pinning point holding back the acceleration of the two Thwaites ice streams [see Tinto & Bell 2011 in the "surge" thread] and is why Goldberg et al 2012 show in the third figure posted here that the ice mass loss in their assumed cavity (the first 55 km of the cavity) slows down after year 20 (which I assume to by 2012) when the cavity runs into the mount.  As the PIG case does not have a submerge mount temporarily pinning its ice stream flow, it might seem that my assumption of using the PIG groundling line retreat response (1996 - 2010) as unfair or biased (ie not accounted for by an adjustment factor less than one in my hazard analysis); however, I have assumed the following (a) advection (see the fourth figure) melts the ice around the base perimeter of the mount while ice thinning and periodic ice stream surges (see the "surge" thread) rapidly degrades the pinning action of the mount by 2015 thus allowing the two Thwaites ice streams to converge into one stream by that data which may allow the ice velocities in the assumed 50-km TG gateway to accelerate up to about 4.5 km/yr by about 2020 (note that the top of the mount is at about El -500m while the top of the assumed cavity by 2012 is also assumed to be near El-500m (see Rignot's figure as the beginning of this thread); therefore, the ice stream near the mount likely does not need to thin too much before the postulated new Thwaites ice shelf floats over the top of the mount thus eliminating this pinning (the same can be said about the two pinning points for the Thwaites Ice Tongue pointed out by Tinto & Bell 2011).
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AbruptSLR

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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #2 on: February 28, 2013, 03:47:48 PM »
Regarding the 1.5 factor that I applied to adjust for both the steep ice slope for Thwaites (than for PIG) and for the stronger basal ice melt stream (particularly for the west Thwaites ice stream), I (a)  first provide the accompanying figure from Gladstone et al 2012 showing how once the temporary pinning point between 100km and 150km (using the coordinates in the figure) that the velocity of the ice at the calving front accelerates rapidly once the grounding line retreats back to the area with a steeper ice surface slope (around 170km) (see the second figure to compare the steepness of the ice surfaces both along the PIG trough and along the TG gateway); thus as the ice surface slope is already as steep as PIG will be after 2040 we need an adjustment factor; and (b) the figure in the immediate previous post regard the advective process indicates that it is driven by the siphon action of the fresh melt water floating up thus suction in more warm CDW to melt more ice (and note that the CDW indicated in this 2D case has the thermal potential to melt more ice if it could access a sufficiently large ice contact area as in this 2D case most of the warm CDW flows back to the sea); however, the presence of the strong basal melt water stream coming from the melt water network beneath Thwaites would provide an induction jetting action to supplement the siphon mechanism this increasing the activity of the advection process with in the Thwaites cavity as compared to the PIG cavity (also note that if methane gas is released by methane hydrate decomposition at the retreating grounding line within a cavity then this would further accelerate the advective process (note the Thwaites gateway has plenty of sediment that could accommodate buried methane hydrates beneath the grounding line for Thwaites).  As with regard to the expected rate of thinning of the newly forming ice shelf for Thwaites please see third accompanying figure showing a current thinning rate of over 3 m per year for the Thwaites (and the PIG) ice shelfs/tongues.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #3 on: February 28, 2013, 04:33:15 PM »
I do not have time to explain the following two figures (for the 2040-2060 timeframe for Thwaites) in detail.  But I will say that in the first figure it is assume that the subglacial cavity previously indicated for Thwaites by the end of 2040 show immediately thereafter intercept the damming ridge and the assumed associated subglacial lake identified by Schroeder, D.M., Blankenship, D.D., Young, D.A. 2013, which it is assumed would have the effect of both: (a) releasing a lot of water from the assumed subglacial lake which accelerate ice flow and ice shelf thinning (possibly in a concentrated surge) and (b) it is assumed would laterally extend the cavity in the east-west directions the full length of the assumed lake behind the damming ridge, which would allow the cavity to then extend in three directions at once (east, west and south), following the bathymetry indicated in the second posted figure.  In follow-up posts I expect to talk about: (a) the possible accelerated calving from this new ice shelf circa 2060; (b) the influence of more frequent surface ice melting around this time frame possibly leading to a melt pond mechanism similar to what has happened in 2002 for Larsen B ice shelf and is soon expected to happen to Larsen C ice shelf as indicated in the third accompanying figure; and (c) El Nino effects, and others effects
« Last Edit: March 10, 2013, 02:24:54 AM by AbruptSLR »
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #4 on: March 01, 2013, 03:49:08 PM »
The first two images of the previous post is a gross over simplification of the sequence of events that I have in mind for the area indicated in transparent orange and labeled Thwaites Ice Shelf (& its indicated extent needs refinement).  This orange area is not intended to represent a continuous ice shelf and is best thought of here as the area from which the grounding line has retreated as indicated in images in the "collapse" thread.  Over the next several days of posts I hope to clarify the various factors, mechanisms, sequences and timing that could possibly lead to such an extensive grounding line retreat in such a relatively short period of time, and with the grounding line retreat following particular pattern of the deep troughs carved into the seafloor during previous interglacial events (such as the Eemian).  I would like to start the series of posts on this matter by postulating that between 2020 and 2040 the calving front of new Thwaites Ice Shelf will undergo a sequences of large calving events similar to that exhibited by the recent PIG Ice Shelf calving event (illustrated by the accompanying image).  I expect that within the next year the portion of the PIG Ice Shelf infront of the large crevasse will have broken free resulting in the rapid retreat of the ice shelf face to a location many km upstream of the face location previously expected by many researchers by this time.  As the postulated new Thwaites Ice Shelf in 2020 is expected to be roughly 50-km wide I expect that such major calving events will happen regularly between 2020 and 2040, which would relieve the buttressing support of the lost portions of Thwaites Ice Shelf to the grounded ice on the sides of the threshold, and thus I expect that by 2040 the Thwaites gateway will have widened from about 50-km to about 80-km (roughly a retreat of 15-km for the face of each gateway sidewall over a 20-year period).
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #5 on: March 01, 2013, 04:09:18 PM »
By 2040 when the postulated threshold Thwaites subglacial cavity is proposed by me to intercept the subglacial lake (indicated by the dashed yellow line in the transparent orange area), I expect that the temporary outburst of high pressure subglacial lake water will cause a surge of ice outflow (into the gateway with a receding ice shelf) which will rapidly thin the grounded ice along the longitudinal axis of the subglacial lake in the east-west direction so that before 2045 the ice streams in the two deep east-wast troughs will be subjected to the "Jakobshavn Effect" instability.  For those unfamiliar with the Jakobshavn Effect", I provide the accompanying two figures and the following Wiki quote discussing this effect and the rapid retreat of the Jakobshavn Glacier from 2000 to 2005:
"Jakobshavn is one of the fastest moving glaciers, flowing at its terminus at speeds of around 20 metres per day. The speed of Jakobshavn Glacier varied between 5700 and 12600 metres per year between 1992 and 2003. The ice stream's speed-up and near-doubling of ice flow from land into the ocean has increased the rate of sea level rise by about 0.06 millimeters (about 0.002 inches) per year, or roughly 4 percent of the 20th century rate of sea level rise. Jakobshavn Isbrae, retreated 30 km from 1850–1964, followed by a stationary front for 35 years. Jakobshavn has the highest mass flux of any glacier draining the Greenland Ice Sheet. The glacier terminus region also had a consistent velocity of 20 meters/day (maximum of 26 m/day in glacier center), from season to season and year to year, the glacier seemed to be in balance from 1955-1985. After 1997 the glacier began to accelerate and thin rapidly, reaching an average velocity of 34 m/day in the terminus region. It also thinned at a rate of up to 15 m/year and retreated 5 km in six years. Jakobshavn has since slowed to near its pre-1997 speed, the terminus retreat is still occurring. On Jakobshavn the acceleration began at the calving front and spread up-glacier 20 km in 1997 and up to 55 km inland by 2003. The position of this calving front, or terminus, fluctuated by 2.5 km around its annual mean position between 1950 and 1996. The first mechanism for explaining the change in velocity is the "Zwally effect" and is not the main mechanism, this relies on meltwater reaching the glacier base and reducing the friction through a higher basal water pressure. A moulin is the conduit for the additional meltwater to reach the glacier base. This idea, proposed by Jay Zwally, was observed to be the cause of a brief seasonal acceleration of up to 20% on the Jakobshavns Glacier in 1998 and 1999 at Swiss Camp. The acceleration lasted 2–3 months and was less than 10% in 1996 and 1997 for example. They offered a conclusion that the "coupling between surface melting and ice-sheet flow provides a mechanism for rapid, large-scale, dynamic responses of ice sheets to climate warming". The acceleration of the three glaciers had not occurred at the time of this study and they were not concluding or implying that the meltwater increase was the cause of the aforementioned acceleration. Examination of recent rapid supra-glacial lake drainage documented short term velocity changes due to such events, but they had little significance to the annual flow of the large outlet glaciers.
The second mechanism is a "Jakobshavn effect", coined by Terry Hughes, where a small imbalance of forces caused by some perturbation can cause a substantial non-linear response. In this case an imbalance of forces at the calving front propagates up-glacier. Thinning causes the glacier to be more buoyant, even becoming afloat at the calving front, and is responsive to tidal changes. The reduced friction due to greater buoyancy allows for an increase in velocity. The reduced resistive force at the calving front is then propagated up glacier via longitudinal extension in what R. Thomas calls a backforce reduction.
This mechanism is supported by the data indicating no significant seasonal velocity changes at the calving front and the acceleration propagating upglacier from the calving front. The cause of the thinning could be a combination of increased surface ablation and basal ablation as one report presents data that show a sudden increase in subsurface ocean temperature in 1997 along the entire west coast of Greenland, and suggests that the changes in Jakobshavn Glacier are due to the arrival of relatively warm water originating from the Irminger Sea near Iceland.
Recent large calving events where the glacier produces icebergs have also been found to trigger earthquakes due to the icebergs scraping the bottom of the fjord."

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AbruptSLR

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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #6 on: March 01, 2013, 05:17:42 PM »
The transparent orange area indicates that I expect the groundling line to retreat in two main branches (one arcing east [along the PNE-TG yellow arc line] and the other a mirror image arcing to thw west) for a distance of about 200 km (along each arc) over an appoximately 20-year period from 2040 to 2060, which is along three times the rate of grounding line retreat that I justified for the 2012 to 2040 period earlier in this thread.  It is noted here that unlike the Jakobshavn Glacier retreat where the calving front retreated 10 km from 2001 to 2004 and then stablized as the Jakobshavn Glacier grounding line retreated up a positive slope (and stabilized for other reasons), for reasons that I will cite in the next post I believe that the Thwaites Glacial ice along the two nominally 200-km arcs is particularly unstable and is likely to retreat this full distance by 2060 as the "Jakobshavn Effect" will be amplified by a series of factors that when multiplied together will account for the expected 300% increase in grounding line retreat rate as postulated to be the case for the postulated 2040 Thwaites situation.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #7 on: March 01, 2013, 08:23:22 PM »
While the "Jakobshavn Effect" approximately doubled the rate of calving face retreat for the Jakobshaven Glacier from 2001 to 2004, I believe that some of the parameters related the the "Jakobshavn Effect" are already considered in my calculation supporting the postulated 100-km retreat of the Thwaites grounding line from 2012 to 2040; therefore, I will assume an additional factor of 1.7 for the "Jakobshavn Effect" for the next postulate 200-km grounding line retreat from 2040 to 2060 (note that I believe that the geometry of the ice streams along these two 200-km arcs are particularly susceptible to the "Jakobshavn Effect" due to the probable steep ice surface gradients from the dropping elevation of the ice surface near the calving from relative to the adjoining ice).  Furthermore, I postulate another 1.1 factor for the increase in CDW temperature by 2050 (assuming RCP 8.5 50%CL radiative forcing). As the rate of basal ice melt due to geothermal heat in the very thin crust in these two 200-km arced trough, I will assume another 1.1 factor.  Furthermore, I assume that by at least 2050 warming of the surface temperature in WAIS will increase the frequency of austral summer ice surface melting (particularly in strong El Nino years), which may introduce the "Zwally Effect" to the Thwaites Basin, for which I apply another 1.1 factor.  To account for the influence of the projected increase in cyclonic activity in this area (including waves, barometric pressure fluctuations and storm surge) I take another 1.1 factor.  Also, by 2050 I believe that the sea ice around Antarctica will begin to melt earlier and will expose more sea water for which I take another 1.1 albedo effect.  Finally, as I believe that methane generated by hydrate decomposition in the exposed seafloor in the transparent orange area, will accelerate advection, I take another 1.1 factor.
Also of these factors taken together: 1.7x1.1x1.1x1.1x1.1x 1.1 x 1.1 = 3.0, which supports the rate of grounding line retreat postulated for these two 200-km arcs by 2060.
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #8 on: March 01, 2013, 08:42:54 PM »
To provide sum support for my 1.1 factor for the "Zwally Effect", I: (a) provide the attached figure of ice surface melt days for the 2004-2005 season, and I postulate that between 2040 to 2060 the surface melt days near the calving fronts for PIG and TG will be similar to that for the Antarctic Peninsula indicated in the figure; and (b) if the ice velocities are as high along these two 200-km long arcs as I postulate for this period then the amount of internal ice melt from internal friction should contribute to the "Zwally Effect".
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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AbruptSLR

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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #9 on: March 01, 2013, 10:49:44 PM »
The attached figure from Levermann et al 2012 indicates that the 50% CL RCP 8.5 projected CDW temperature for 2060 is about 0.15 C above the 2040 temperature thus supporting my 1.1 acceleration factor on grounding line retreat over this time period.
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #10 on: March 02, 2013, 01:57:45 AM »
The first figure shows that cyclones in the seas offshore WAIS are becoming more intense (lower central pressure) and more frequent (higher density); while the second figure indicates that both strong La Nina and strong El Nino events drive rossby wavetrains directly towards the ASE which is significant as strong ENSO events have been projected as global warming progresses.  Thus both of these figures support my 1.1 factor for storm and wave effects on grounding line retreat for TG.
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #11 on: March 02, 2013, 02:05:50 AM »
The WAIS-Divide Ice-Core was taken near to the transparent orange area and the following public statement about the measured basal ice melting rate at the WAIS Divide ice-core location supports the 1.1 factor that I used for grounding line retreat in this area:

"High Basal Melt at the WAIS-Divide ice-core site
T.J. Fudge, Gary Clow, Howard Conway, Kurt Cuffey, Michelle Koutnik, Tom Neumann,
Kendrick Taylor, and Ed Waddington
We use the depth-age relationship and borehole temperature profile from the WAIS-Divide ice
core site to determine the basal melt rate and corresponding geothermal flux. The drilling of the
WAIS-Divide ice core has been completed to 3400 m depth, about 60 m above the bed. The age
of the deepest ice is 62 ka, younger than anticipated, with relatively thick annual layers of ~1 cm.
The borehole temperature profile shows a large temperature gradient in the deep ice. We infer a
basal melt rate of 1.5 (±0.5) cm yr-1 using a 1-D ice flow model constrained by these data sets.
The melt rate implies a geothermal flux of ~230 mW m-2, three times the measured value of 70
mW m-2 at Siple Dome.
We compile radio-echo sounding data sets to assess the spatial extent of high melt. Deep internal layers are the most useful for inferring spatial patterns of basal melt. Unfortunately, the
IceBridge WAIS-core flight and two site-selection surveys did not image consistent reflectors
deeper than Old Faithful (2420 m and 17.8 ka). A ground-based survey by CReSIS (Laird et al.,
2010) was able to image consistent layers as deep as 3000 m, but the survey is not oriented along the ice-flow direction making interpretation more difficult. There is no obvious draw down of
deep internal layers that would indicate an area of localized melt. While this suggests a uniform
melt rate within the survey, it might also indicate that other factors (e.g. accumulation gradients,
rough bed topography) obscure the influence of basal melt on the internal layer depths."
« Last Edit: March 02, 2013, 02:15:52 AM by AbruptSLR »
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #12 on: March 02, 2013, 03:05:02 PM »
It would appear that the heart of my arguement against Pfeffer et al 2008's supposed limit on ice export out of their assumed 120 sq km gateway at an assumed "upper bound" average ice export velocity just under 15 km/yr boils down to the following hazard analysis sequence (already presented) and ice flow activation mechanisms (see all previously discussed multipliers to the base PIG grounding line retreat rate from 1996 to 2010, including most significantly: advection, CDW temperature, basal meltwater network, and the Jakobshavn Effect) not previously considered by Pfeffer et al 2008 (or the NRC 2012 or the IPCC WG1 AR5 SOD, see the "Critique" thread):
(1) First the following sequence discussion is intended to clarify how the hazard scenario for the period from 2012 to 2040 sets-up the case for the "Jakobshavn Effect" for the period from 2040 to 2060: (1a) First I will note that Pfeffer et al 2008's ice velocity limit of about 15 km/yr by 2100 assumes a linear acceleration from about 2 to 2.5 km/yr for a beginning year of about 2005 and that basal friction is involved/contolling during this whole period, while I assume that for the period from 2012 to 2040 that the combination of advective subglacial cavity formation and periodic (quasi-annual) surges thin the new Thwaites ice shelf thins sufficiently (in the about 100km distance from the submerge mount to the ridge dammed lake) so that this ice shelf is not "pinned" on any seafloor features and so that the side shear on the ice shelf is progressively being degraded to offer buttressing support from the new ice shelf so that by 2040 the average floating ice export velocity through the Thwaites gateway is about 4 to 5 km/yr; and (1b) By 2040 when the subglacial cavity intercepts the subglacial lake behind the damming ridge that the associated major surge event further thins the new Thwaites Ice Shelf and degrades the side shear resistance on the shelf (by ice fracturing) that the Jakoshavn Effect instability can rapidly propagate upstream along the two 200-km long arcs previously discussed along the deep two east-west troughs.  I will continue my second point as to why the Jakoshavn Effect is so unique for the Thwaites case that I propose (that if confirmed) it should be called the "Thwaites Effect"
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #13 on: March 02, 2013, 03:51:06 PM »
This discussion continues directly from my previous post, elaborating on both the Jakobshavn Effect and why the Thwaites geometry/condition is so unique that along the indicated ice streams/troughs (east-west 200-km long arcs) that the ice velocities will accelerate into a "Thwaites Effect":
(2)  First the readers may want to review the attached pdf elaborating on the theory of the Jakobshavn Effect, where it is possible that a warm ocean water advection sufficiently thinned the Jakobshavn Ice Tongue rapidly that this created a dynamic out of balance force equilibrium instability that was rapidly propogated upstream resulting in a relatively rapid grounding line retreat and rapid doubling of the average ice export velocity.  For the "Thwaites Effect" I assume that the major surge from the lake water outburst circa 2040 leads to a reduction of lateral support from the ice shelf that would be propogated rapidly upstream along the two east-west arcs due to a combination of: continued undermining of the glacial ice at the tip of the subglacial cavities, rapid basal ice melting due to high geothermal effects shown by the WAIS-Divide ice-core, steep ice surface slopes resulting in high driving forces, and decreasing ice viscosity parameter primarily due to the increase in internal ice friction as the glacier ice stream velocities along the east-west troughs have been steadily increasing from 2012 through 2060; and with regard to the numerical relationships of the ice viscosity parameter, basal friction and driving stress upstream from the grounding line see the accompanying Figure 8 from Van der Veen et al 2011 [see attached pdf].  Figure 8 shows that depending on the details of the dynamics it is theorically possible for the ice stream velocities (of the ice immediately upstream of the grounding line) to accelerate from 4km/yr upto (in extreme cases) 50 km/yr within the limits of the ice stream trough, while down steam of the grounding line the continued thinning of the new Thwaites ice shelf (for parallel cases see how the Filchner-Ronne Ice Shelf and the Ross Ice Shelf thin in the third attachment) not only keeps this exporting ice from interacting (which might otherwise slow down the ice export velocity and allow the ice shelf to buttress the glacial ice velocity and impede the "Jakobshavn Effect" at the grounding line) with bottom features and further accelerating the thinning ice shelf ice velocity (in the fourth attachment see how the ice shelf velocities for both the Filchner-Ronne Ice Shelf and the Ross Ice Shelf accelerate as the ice shelf thins due to continued subice shelf drag/melting from the existing advective water transferring momentum from the existing water into the ice shelf) as it exists out of the assumed gateway width of about 80km by 2040 (note that (i) the two east-west ice stream flows are assumed to be about 40km wide until they converge to flow north through the 80-km wide Thwaites gateway, and (ii) the retreat of the grounding line from the subglacial lake location to the south is assume to proceed relatively slowly from 2040 to 2060 due to a combination of blocking of ice export from the two major eas-west ice streams, rougher bottom features towards the south, and reduced negtive bottom gradient towards the south).  I expect to elaborate on the likelihood of this hazard scenario and "Thwaites Effect" in my next post later today.
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #14 on: March 02, 2013, 07:02:58 PM »
The two points made in this post provide some supporting evidence as to why the hazard analysis presented so far in this thread (on the proposed PIG/Thwaites system collapse initiation scenario from 2012 to 2060) may be supported by selected historical evidence and by the bathymetric pattern left on the seafloor from the last Eemian peak :
-   The Byournoy-renn Ice Stream (see the first attachment), and the Hudson Strait Ice Stream (see the second attachment, and note that the indicated 50-year peak ice velocity pulse of about 4.5 km/yr flowed through a much wider gateway than is the case for Thwaites but that the water depths of the two gateways are similar), exhibited periods of 50 to 100-years where their ice velocities rapidly accelerated and then rapidly slower-down (see the third attachment) when their grounding lines retreated down a reverse/negative marine seafloor slope.  My collapse initiation scenario postulates that the Thwaites Glacier (which has comparable geometries to both the Byournoy-renn, see the first attachment to this post), rather than rapidly slowing-down as occurred in the Byournoy-renn, and the Hudson Strait, Ice Stream cases (see third attachment), when their (Byournoy-renn & Hudson Strait) grounding line retreat beyond the reverse/negative slope area back into ascending/positive slope regions.  Due to the extreme depth of the BSM troughs the length of reverse/negative slopes for the Thwaites east-west 200-km long ice stream arcs, the ice stream velocities get high enough to decrease primarily the ice viscosity parameter sufficiently (and also for geothermal basal ice melting to decrease basal friction sufficiently) so that the Thwaites ice stream continues to accelerate by means of the "Thwaites Effect(Jakobshavn Effect) to at least 2060 (and beyond for the eastern ice stream branch as will be discussed in the next thread).  It is also noted here that as in the case of the Byournoy-renn record, the lateral force required to turn the two ice stream arcs toward the gateway (in the case of Thwaites the lateral force required to turn the two east-west ice streams toward the north) serves as a buttress to the ice streams seeking to flow directly (without turning) out of the gateways, until the ice flow from the arcing ice streams slows sufficiently to allow the direct flow paths to gradually take over.
-   While the precision of historical record is subjected to meaningful degree of uncertainty, taken cumulatively I believe that is provide considerable evidence of abrupt sea level rise for periods of at least 50 to 100 years associated with the collapse of past marine ice sheets (see the discussion in the "collapse" thread) including during the Eemian peak (MIS 5e, or LGM) where the fourth figure indicates possible surges in sea-level during the Eemian peak that could only be associated with the rapid collapse of the WAIS.  Also note that the ice scour and associated sediment deposits associated with the Byournoy-renn Ice Stream indicate very high ice flow velocities. Furthermore, I  believed that a more detailed examination of historical Dansgaard-Oeschger Events, and Bond Events (during the Holocene including the sustained 4m/century SLR rate sustained during Melt-Pulse 1A for five centuries) may help to characterize the sensitivity of marine ice sheets such as the WAIS to sudden collapse due to various triggering parameters.  Such an investigation may further help to identify the role played by bottom topography/bathymetry on the dynamics of marine ice sheet collapse.
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #15 on: March 02, 2013, 07:23:38 PM »
Also briefly in regard to related (but indirect) historical evidence I would like to point out that:
- The Larsen B ice shelf survived during the Eemian peak, and it's collapse in 2002 raises the prospect that the current anthropogenic forcing conditions may degrade the Antarctic ice even more quickly than occurred during the Eemian peak.
- The fact that the WAIS-Divide ice-core contained at most 62k old ice (taken with 60 m of the BSB seafloor) demonstrates conclusively that the Thwaites glacial ice was not there during the Eemian peak (however, this does not necessarily mean that the WAIS ice mass loss all occurred within one hundred years as I am contending is physically possible), as previously indicated by Strugnell et al. 2012 observation that the modern distribution of the Antarctic adult Turquet's octopuses provides biological evidence that the Weddell, the Bellingshausen, the Amundsen and the Ross Seas were all interconnected at the peak of the Eemian.
- Muhs et al. 2012 (and other studies) present physical evidence from several sites around demonstrate that during Eemian peak/MIS 5.5 RSLR (including a RSLR of over 6 m offshore of California) was sufficiently high that the WAIS clearly melted-out during the Eemian peak.
- Recent (2012) ice cores from the GIS indicate that during the Eemian peak that the Antarctic must have contributed from 3.4 to 3.8 m to eustatic SLR (most of which must have come from the WAIS).
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #16 on: March 02, 2013, 11:46:33 PM »
Before opening a new thread on the "Collapse Main Period from 2060 to 2100" I would like to make couple of more comments about the transparent orange area labelled ice shelf previously: (a) the reverse/negative slope along one of the arced ice stream flows needs to rise from El-2000m to El -500 within a distance of approximately 300km, or 5m/km, which as indicated by the PIG case is not difficult for advective sub-ice shelf melting to achieve (without the ice shelf re-grounding) for a rate of grounding line retreat such as that for PIG (retreating 25 km in 16 years or about 1.6km/yr ); however, for the period from 2040 to 2060 I have stated that for the Thwaites case the grounding line will be retreating at a rate of approximately 200km in 20years of 10 km/yr, which might be difficult for advective sub-ice shelf melting to achieve; however, (b) after 2040 I have postulated that the "Jakobshavn Effect" will take hold, which should result in more of an "Ice Melange" rather than an "Ice Shelf" (see the attached image for the distinction) in the transparent orange area; which may cause some entrained icebergs within the Ice Melange to bump along the seafloor (causing mini-ice earthquakes of magnitude 2 or so) but given the expected driving force of the ice melange flow such seafloor bumping events are not expected to pin such icebergs entrained with the ice melange flow.
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #17 on: March 08, 2013, 12:16:16 AM »
I don't believe that I have shown the attached figure before, but it helps to clarify how the Belgica Trough focuses warm CDW towards the Ferrigno Glacier, which inturn should lead to the formation of a subglacial cavity below the Ferrigno Glacier, which should result in the rapid retreat of the groundling line for the Ferrigno Glacier down to the floor of the rift valley that it rests in by 2060.  Also, note that this rift valley leads directly to the upstream end of the PIG drainage basin, and thus the degradation of the Ferrigno Glacier will contribute to ice mass loss from the PIG drainage basin.
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #18 on: March 08, 2013, 07:24:25 PM »
If it is not clear to some readers what type of major calving events that could occur within the Thwaites ice drainage basin during the period from 2040 to 2090, that could result in an ice melange in the transparent orange shaded areas, then I recommend that they watch the much viewed youtube clip from "CHASING ICE" showing the largest glacier calving event (in Greenland) ever filmed.

http://www.youtube.com/embed/hC3VTgIPoGU?rel=0


« Last Edit: March 08, 2013, 10:48:52 PM by AbruptSLR »
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #19 on: March 08, 2013, 08:50:58 PM »
I add the ted talk of James Balog for those who don't know about it !

http://www.ted.com/talks/lang/en/james_balog_time_lapse_proof_of_extreme_ice_loss.html

Laurent

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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #20 on: April 17, 2013, 06:34:27 PM »
I make this post in this thread as a continuation of the discussion (as of yesterday) in the "Surge" thread, but I plan to focus here on possible event from 2013 moving forward (to 2060).

First, I would like to note that for the past many years it is likely that the Thwaites Ice Tongue does not provide any significant pinning/buttressing action as indicated by the following quote from MacGregor et al 2012:

"Thwaites Glacier Tongue is not believed to buttress Thwaites Glacier significantly (Rignot, 2001), which is consistent with the muted recent acceleration of the grounded ice directly upstream of the ice tongue (up to 16% between 1992 and 2007; Rignot, 2006, 2008) compared with other glaciers in the ASE (63–108% over the same period), despite similar observations of marginal rifting and terminus retreat.  The terminus advance rate did not change significantly after the 2010 calving event, which removed most of the remaining ice tongue; this observation supports the earlier calculations of its limited buttressing effect."

To me this implies that for the past several years the increasing velocities of the Thwaites Glacier ice streams have not been due to a reduction of the buttressing action from the degrading Thwaites Ice Tongue, but instead possibly due to an activiation of a subglacial lake about 100km south of the current grounding line; resulting in an increase in basal lubrication.  The activiation of the subglacial lake is supported by the location of maximum ice mass loss identified by the GRACE satellite {however, note that if meltwater is flowing out of this subglacial lake, then the GRACE data needs to be corrected for: (a) local glacial isostatic adjustment due to magma backfilling under this very thin crust at this location; and (b) local snow fall in this location [replacing dense meltwater with less dense snow] that might partially mask (both from GRACE and altimeters) the acceleration of glacial ice mass loss}.

Thus if it is the case that a significant portion of the ice mass loss of the Thwaites Glacier is due to the outflow of basal meltwater from a subglacial lake, then this would mean that some relatively large subglacial cavity already exists over the length of the 100km long Thwaites Gateway, with freshwater currently flowing out of this 100km long conduit/cavity.  However, the advection of CDW through a large cavity in the Thwaites trough, could be accelerated by a jet of fresh meltwater (from the drainage of the subglacial lake at the south end of the 100km long gateway); and if so this could facilitate the acceleration of the retreat of the grounding line from the south end of the trough down into the BSB.
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #21 on: April 17, 2013, 09:45:16 PM »
Article on PIG in cryosphere discuss, kilometer scale resolution. 87Km^3 /yr over 2008-2011. Nice pics

http://www.the-cryosphere-discuss.net/7/1591/2013/tcd-7-1591-2013.html

sidd

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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #22 on: April 22, 2013, 01:47:19 AM »
The attached figure showing the average snowfall accumulation across Antarctica from 1955 to 2005.  This data clearly indicates that the area at the south end of the Thwaites gateway is one of areas that receives the most snowfall anywhere in Antarctica; however, as discussed in the "Surge" thread the GRACE satellite indicates that this same area has the highest rate of ice mass loss of anywhere in Antarctica.  This clearly indicates that if one were to subtract out the mass of snowfall in this area one would then calculate a higher rate of acceleration of glacial/meltwater ice mass loss from this area than would would calculate from the uncorrected GRACE reading; which implies that future ice mass loss from this area are likely to be higher than current projections based on the uncorrected GRACE data.
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #23 on: April 23, 2013, 01:02:49 AM »
In the following AGU abstract from 2012, DeSanto et al postulate that geothermal energy from the active volcano Mount Takahe may be accelerating ice flow along on of the seven tributary ice streams feeding into the Thwaites Glacier.  The attached figure from MacGregor et al 2009 showing lateral shear strain clearly indicates moderate ice flow from the tributary ice stream passing by the base of Mt Takahe.

C31A-0576: Evaluating transience of a potential geothermal heat flux anomaly beneath a tributary ice stream of Thwaites Glacier, West Antarctica
Authors: John B DeSanto, Donald D Blankenship, Duncan A Young, Luc L Lavier, Eunseo Choi
Author Institutions: Institute for Geophysics, University of Texas, Austin, TX, USA
The Amundsen Sea Embayment of the West Antarctic ice sheet (WAIS) is currently one of the most rapidly changing sectors of a continental ice sheet. As a marine ice sheet, the WAIS is in a potentially unstable configuration. In addition to known active volcanoes such as Mt. Takahe and Mt. Murphy, subglacial volcanic activity has been identified using ice layer drawdown anomalies. Drawdown anomalies are features identifiable by a characteristic radar signature and represent significant loss of basal ice. We identify several features with the geometry of drawdown anomalies in the Thwaites Glacier along an ice stream tributary near Mt. Takahe. By modeling the flow of ice along the ice stream, we assess the hypothesis that these drawdown anomalies are a coherent feature caused by basal melt that is consistent with subglacial volcanic activity. The melt rate is then used to determine the spatial and temporal variations of geothermal heat flux in the region. We discuss these variations in the context of their geologic, morphologic and glaciologic setting and their implications for local volcanism and its impact on ice flow.
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #24 on: April 23, 2013, 03:25:38 PM »
This following summary regarding the current version of the PISM glacial model (and the two attached figures) indicate that: (a) older glacial models project less ice mass loss than does the PISM model that considers enthalpy; and (b) the liquid fraction with ice streams (including in the Pine Island Glacier and the Thwaites Glacier, Antarctica) play an important role with regard to both ice viscosity and basal melt water:
An enthalpy formulation for glaciers and ice sheets
By Andy ASCHWANDEN et al, Journal of Glaciology, Vol. 58, No. 209, 2012 doi: 10.3189/2012JoG11J088 441
"Polythermal glaciers contain both cold ice (temperature below the pressure-melting point) and temperate ice (temperature at the pressure-melting point). This poses a thermal problem similar to that in metals near the melting point and to geophysical phase-transition processes in mantle convection and permafrost thawing. In such problems the part of the domain below the melting point is solid while the remainder is at the melting point and is a solid/liquid mixture.  Generally, the liquid fraction of that mixture may flow through the solid phase. For ice specifically, viscosity depends both on temperature and liquid water fraction, leading to a thermomechanically coupled and polythermal flow problem."
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #25 on: May 01, 2013, 03:03:55 PM »
The attached figure is from: SSALTO/DUACS User Handbook: (M)SLA and (M)ADT Near-Real Time and Delayed Time Products
Reference : CLS-DOS-NT-06-034
Nomenclature : SALP-MU-P-EA-21065-CLS
Issue : 3rev 4
Date : 2013/01/29

This Handbook contains the following statements that are relevant to the figure:
"Since February 6th, 2012, Cryosat-2 mission has been integrated in the sytem. This mission is dedicated to the observation of the floating sea-ice as well as the continental ice sheets, but all data acquired over ocean are valuable for the observation of oceanic circulation and mesoscale variations. This major change is the result of the long-standing and fruitful partnership between ESA and CNES and a response to the request from scientific and operational oceanography users. The integration of Cryosat-2 impacts the delivering of Near real time and Delayed time Sea Level Anomalies (SLA) and maps of SLA (MSLA)."
"A Cryosat-2 Processing Prototype (C2P) (described in Boy et al, 2011) has been developed on CNES side to lay the ground for various SAR processing studies. The processing chains ingest Level-0 telemetry files distributed by ESA, and perform the following steps to generate Sea Level Anomalies (SLA) values for each altimeter measurements:
- Level-1: Decommutation, time-tagging and localization of measurements
- Level-1b: Calculation of instrumental corrections and geophysical/meteorological corrections
-  Level-2: MLE4 waveforms Retracking and calculation of SLA
The prototype processes data almost continuously over ocean, either in Low Resolution Mode (LRM) or in the Doppler/SAR mode processed as pseudo-LRM mode allowing to increase the coverage (figure 2)."

This figure clear shows the very high (up to positive 0.3 meters in the ASE) SLA, from 2011 to 2012, all along the coastline of West Antarctica; very possibly due to upwelling of warm CDW along these coastlines.  This very high SLA in the Amundsen Sea Embayment (ASE) can directly contribute to destabilizing the PIG and Thwaites glaciers.
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #26 on: May 01, 2013, 03:04:41 PM »
The attached figure is from: SSALTO/DUACS User Handbook: (M)SLA and (M)ADT Near-Real Time and Delayed Time Products
Reference : CLS-DOS-NT-06-034
Nomenclature : SALP-MU-P-EA-21065-CLS
Issue : 3rev 4
Date : 2013/01/29

This Handbook contains the following statements that are relevant to the figure:
"Since February 6th, 2012, Cryosat-2 mission has been integrated in the sytem. This mission is dedicated to the observation of the floating sea-ice as well as the continental ice sheets, but all data acquired over ocean are valuable for the observation of oceanic circulation and mesoscale variations. This major change is the result of the long-standing and fruitful partnership between ESA and CNES and a response to the request from scientific and operational oceanography users. The integration of Cryosat-2 impacts the delivering of Near real time and Delayed time Sea Level Anomalies (SLA) and maps of SLA (MSLA)."
"A Cryosat-2 Processing Prototype (C2P) (described in Boy et al, 2011) has been developed on CNES side to lay the ground for various SAR processing studies. The processing chains ingest Level-0 telemetry files distributed by ESA, and perform the following steps to generate Sea Level Anomalies (SLA) values for each altimeter measurements:
- Level-1: Decommutation, time-tagging and localization of measurements
- Level-1b: Calculation of instrumental corrections and geophysical/meteorological corrections
-  Level-2: MLE4 waveforms Retracking and calculation of SLA
The prototype processes data almost continuously over ocean, either in Low Resolution Mode (LRM) or in the Doppler/SAR mode processed as pseudo-LRM mode allowing to increase the coverage (figure 2)."

This figure clear shows the very high (up to positive 0.3 meters in the ASE) SLA, from 2011 to 2012, all along the coastline of West Antarctica; very possibly due to upwelling of warm CDW along these coastlines.  This very high SLA in the ASE can directly contribute to destabilizing PIG and Thwaites Glacier.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #27 on: May 05, 2013, 08:35:28 PM »
I thought that I would post this image from NASA's QuikScat satellite detected extensive areas of snowmelt, shown in yellow and red, in west Antarctica in January 2005.  The 2005 melt was intense enough to create an extensive ice layer when water refroze after the melt. However, the melt was not prolonged enough for the melt water to flow into the sea.

A NASA scientist said "Water from melted snow can penetrate into ice sheets through cracks and narrow, tubular glacial shafts called moulins. If sufficient melt water is available, it may reach the bottom of the ice sheet. This water can lubricate the underside of the ice sheet at the bedrock, causing the ice mass to move toward the ocean faster, increasing sea level."

While such melting periods are episodic, such melting events should become more frequent with global warming (particularly during significant El Nino events) and are likely to have a major impact on the Thwaites drainage basin.
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #28 on: May 10, 2013, 12:03:48 AM »
I thought that I would post the attached figure that shows the change in ocean bottom pressure (in cm of water head) as an indication of water mass distribution in the ocean.  This image from the GRACE satellite for December 2012 would area to indicate a relatively high water mass loss from around Greenland and a relatively high addition of water mass around the Bellingshausen and ASE for last December.  While this ocean bottom pressure distribution is transient (with time); still this transient ocean water pressure off the coast from the PIG and the Thwaites Glacier could have helped to accelerate (at least temporarily) ice mass loss from these key ice features.
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #29 on: May 10, 2013, 10:51:28 PM »
To me the attached figure (from the University of Southern Florida) indicates: (a) a substantial amount of heat has entered the ocean during the El Nino hiatus period; and (b) the past trend of ice mass contribution to SLR has been roughly linear for the past 10-years, unless the El Nino hiatus effect is depositing more ocean water on land than is running back into the ocean (thus potentially masking any acceleration in SLR due to ice mass loss).  Again I would like to point out that much of the heat content going into the ocean eventually makes it into the Southern Ocean where an end of the El Nino hiatus would mean an increase in upwelling, which would deliver the extra heat in the CDW to the grounding lines of ice features around much of Antarctica.
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #30 on: May 10, 2013, 11:08:59 PM »
Am I seeing an increase of 1cm between 2011 and 2012 and probably the same amount in 2013 !?

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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #31 on: May 10, 2013, 11:31:01 PM »
Laurent,

As confirmed by the attached image (to April 2013) from the Aviso website (http://www.aviso.oceanobs.com/en/news/ocean-indicators/mean-sea-level/), yes roughly what you are seeing from the USF image is correct.  However, Aviso currently only cites a long term SLR slope of 3.18 mm/yr (rather than 20 to 25 mm/yr); because during the El Nino hiatus period the pothole in sea level from roughly 2010 to 2012 occurred to more preciptation falling on land than was returning to the sea.  Thus we will need to watch to see if a high SLR trend line slope continues into the future, or reverts to lower levels again.
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #32 on: May 12, 2013, 02:33:23 AM »
From an article at:
http://www.realclimate.org/index.php/archives/2013/04/ice-hockey/
Eric Steig states:
"Looking at the very long term record from the WAIS Divide ice core, it appears that similar conditions could have occurred about once per century over the last 2000 years. Hence our answer to the question, “are the observations of the last few decades a harbinger of continued ice sheet collapse in West Antarctica?”, is tentative: “Probably”."

Eric Steig also provides the accompanying figure and the related statements:
"Figure 1. (a) Comparison of averaged δ18O (blue) across West Antarctica with the recent temperature record of Bromwich et al. (2013) from central West Antarctica (yellow). The light blue background is the decadal smoothed values +/- 1 standard error assuming Gaussian statistics. (b) Number of records used, and probability that the decadal average is as elevated as the 1990s (green).
Data sources: Most of the data for this figure have been available at http://nsidc.org/data/NSIDC-0425.html for some time. There’s a new location (which will link to the old one) where more recent data sets will be placed, but it’s not all up yet: http://nsidc.org/data/nsidc-0536.html.
Our results show that the strong trend in δ18O in West Antarctica in the last 50 years is largely driven by anomalously high δ18O in the most recent two decades, particularly in the 1990s (less so the 2000s). This is evident in the temperature data as well (top panel of the figure). The 1990s were also very anomalous in the tropics — there were several large long-lived El Niño events with a strong central tropical Pacific expression, as well as only very weak La Niña events. As in the tropics, so in West Antarctica: the 1990s were likely the most anomalous decade of the last 200 years."
Here I note that the longer the current El Nino hiatus period lasts, the more likely the next El Nino cycle will exceed that from the 1990's period; with potentially serious consequences for the WAIS ice mass loss.
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #33 on: May 13, 2013, 04:28:05 AM »
The attached image from a NASA/Goddard model simulation of the change in ice elevation from 2002 to 2011 shows upward of 60m of elevation for a portion of the ASE glaciers (not the PIG is on the east (left) side of this image.
« Last Edit: May 13, 2013, 04:46:31 AM by AbruptSLR »
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #34 on: May 26, 2013, 07:48:36 PM »
I provide the following relevant abstracts from the Nineteenth Annual WAIS Workshop, 2012 (note that the first abstract is particularly relevant to the potential widening of the Thwaites Gateway towards the eastside as noted by MacGregor et al 2013):

1. "Thermal migration of ice stream shear margins
C. Schoof and M. Haseloff

Ice stream shear margins can be viewed as boundary layers connecting a Poiseuille-like shear flow in ice ridges with a membrane-like, lateral-shear dominated flow in the ice stream itself.  The discharge of the ice stream is then highly sensitive to its width: with a Glen's law rheology, ice velocity scales as the fourth power of ice stream width. A crucial question therefore is how the width of the ice stream evolves over time.  Existing, depth-integrated models of ice stream dynamics typically predict that the bed underlying an ice ridge should freeze over time, while the ice stream bed remains unfrozen, and the transition between the two should occur in the shear margin. Depth-integrated models however cannot describe the details of that transition, which would allow the rate of margin migration to be computed.  We consider this boundary layer problem in detail, focusing on an abrupt transition from free slip to no slip at the point where the bed temperature changes from temperate (i.e., at the melting point) to subtemperate (i.e., below the melting point). This engenders multiple singularities in both, stress field and hence volumetric heating rate, and in heat flux. We show that the strength of these singularities is controlled by the far field, and that one of the singularities in the heat flux must be alleviated in order to allow the ice stream to widen. In the process, we show that at least a small zone of temperate ice must also form above the transition between frozen and unfrozen ice.  We show that the alleviation of the heat flux singularity is possible only for specific combinations of the following quantities: i) the strength of shear heating in the margin dictated by lateral shear stress acting on the ice stream margin ii) the background temperature gradient dictated by surface temperatures and advection in the ice ridge and iii) the margin migration rate.  More specifically, in the absence of significant advection from the ice ridge, we are able to show (by using the Wiener-Hopf technique) that margin migration rate is determined uniquely by lateral shear stresses and background temperature gradient."

2. "The Losers Next Door: Mass loss from Thwaites, Pope and Smith glaciers
Ben Smith, Ian Joughin, and David Shean

Headlines about mass loss from the Amundsen Sea sector are often dominated by the antics of Pine Island glacier. But just next door, three large glaciers have each made their own contributions to sea level. A synthesis of laser-altimetry and photogrammetry from ICESat, IceBridge, and Worldview, shows that Thwaites, Pope and Smith have together lost more mass since 2009 than PIG, and while the near-grounding-line thinning on PIG appears to be thinning, it has held steady over the last year on Smith Glacier. The cause of the large ice losses in these glaciers is probably ongoing changes near the grounding line. Visible-light and radar imagery reveals changes in crevassing patterns and in the configuration of ice rises near the fronts of all of these glaciers, suggesting that thinning ice shelves have lost some support from submarine peaks that once helped buttress them against ice flowing form upstream, while combined altimetry and ice-sounding measurement reveal changes in the extent of grounded ice.  At the same time, melt near the grounding lines has eroded contact between ice and rock. In some cases, the changes have been subtle, as in the Thwaites Ice Shelf, where the freeboard of nearly-floating ice has decreased, leading to patchy flotation; in the case of Pope Glacier, the change is not subtle at all: beneath the fastest-flowing part of the glacier, the ice has thinned by nearly 30 m/yr since early measurements in 2002, creating a dramatic new embayed area upstream of the grounding line. The extent to which these changes can continue will depend greatly on the future rate and pattern of marine melt, the specifics of which will be discussed in a companion presentation by Ian Joughin."

3. "Model--‐Based Analysis of Ice Sheet Thinning in the Amundsen Sea Embayment
Ian Joughin and Ben E. Smith

Strong thinning as ice streams have sped up along the Amundsen Coast produces ice loss well in excess of that from other regions of Antarctica. Much of the increases in speed appear to be caused by the loss of buttressing as ice shelves have thinned in response to warmer ocean water and subsequent loss of basal traction as the grounding line has retreated. We examine this response for Pine Island and Thwaites glaciers using models constrained by satellite data. Our earlier work reproduced the transient response on Pine
Island Glacier and predicted that strong near thinning near the grounding line should abate, but that overall losses should remain high as thinning diffuses inland. Here we find that this conclusion is supported by new IceBridge data, which show recent reduction of near grounding line thinning as speeds have leveled off. On Thwaites Glacier, we conducted a series of numerical experiments to investigate sensitivity of ice flow to ice-shelf loss and grounding-line retreat. The model suggests that recent changes in speed are the result of enhanced rifting that weakened the ice shelf followed by retreat of the grounding line. In response, surface slopes have thinned causing the speedup to migrate inland. We also use a prognostic model to investigate whether such thinning will continue over the next century."
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #35 on: May 27, 2013, 03:22:19 AM »
From the INSTAAR University of Colorado, Boulder, website, regarding the WAIS Divide Core:

"The project as been ongoing since the WAIS field site was established in 2005, and during 5 subsequent (short) austral summer field seasons of drilling, over 4,405 meters of ice have been recovered. The cores are allowing scientists from many different universities and research groups to apply their individual measurement expertise to extract the highest resolution climate record ever created for a polar ice core. The WAIS Divide climate records have an absolute, annual-layer-counted chronology for the most recent ~40,000 years. It was expected that the lower temporal resolution records would extend beyond ~100,000 years before present, but it was a big surprise to many that the oldest ice at the bottom of this core was less than 70,000 years old. Basal melting has played a role in removing ice from the bottom of the ice core."

This quote implies that over the last approximately 100,000 years, basal melting has removed over 1,300 meters of ice from the bottom of the WAIS Divide location (assuming that the scientist's expectation to find approximately 100,000 year old ice at the bottom of the core is correct).
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #36 on: May 27, 2013, 03:28:07 AM »
The attached figure from Nature Geoscience 2012, DOI: 10.1038/NGEO1671, show in good detail the areas of the Antarctic subject to the indicated number of days of surface ice melting in January 2005.  This figure indicates that both the PIG/Thwaite drainage basins and the Ross Sea Embayment areas are subject to a substantial risk of surface ice melting in the future.
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #37 on: June 22, 2013, 07:36:48 PM »
From the Center for Remote Sensing of Ice Sheets (CReSIS):

https://www.cresis.ku.edu/

A seismic investigation of the subglacial environment along Thwaites Glacier, West Antarctica
By: Leo E. Peters, Sridhar Anandakrishnan, Richard B. Alley, Huw J. Horgan, Joseph A. MacGregor, Anthony, M. Hoch, Donald E. Voigt

The first image shows the area of the seismic investigation of TG (circa 2009).

The second image shows on overview of the observed basal shear stress in the study area of TG.  I would like to note that in the areas of high basal shear stress, where the ice velocities are also relative high (such as in the Thwaites Glacier, TG, gateway near the submerged mount that is helping to pin TG; that the frictional heat associated with this basal shear stress can be sufficient to melt a portion of the basal ice; which inturn can contribute to the accumulation of basal meltwater in associated subglacial lakes and/or drainage systems.

The third image shows a close-up of the portion of the study area shown in the second image, indicating the lines and selected parameters of the seismic investigation (circa 2008-2009).

The fourth image shows the investigators interpretation of the bed conditions along one of the lines of investigation shown in the third image.
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #38 on: June 29, 2013, 04:50:27 PM »
The following paper points out that when the Amundsen Sea sector ice mass change measurements are corrected for the condition of the firn then the corrected measurements indicate more ice mass loss than previously reported:

Continuously accelerating ice loss over Amundsen Sea catchment, West Antarctica, revealed by integrating altimetry and GRACE data
By: Hyongki Lee; C.K. Shum; Ian M. Howat; Andrew Monaghan; Yushin Ahn; Jianbin Duan; Jun-Yi Guo; Chung-Yen Kuo; Lei Wang

Earth and Planetary Science Letters; Volumes 321–322, 1 March 2012, Pages 74–80

"Abstract
Satellite altimetry and Gravity Recovery and Climate Experiment (GRACE) measurements have provided contemporary, but substantially different Antarctic ice sheet mass balance estimates. Altimetry provides no information about firn density while GRACE data is significantly impacted by poorly constrained glacial isostatic adjustment signals. Here, we combine Envisat radar altimetry and GRACE data over the Amundsen Sea (AS) sector, West Antarctica, to estimate the basin-wide averaged snow and firn column density over a seasonal time scale. Removing the firn variability signal from Envisat-observed ice-sheet elevation changes reveals more rapid dynamic thinning of underlying ice. We report that the net AS sector mass change rates are estimated to be − 47 ± 8 Gt yr− 1 between 2002 and 2006, and − 80 ± 4 Gt yr− 1 between2007 and 2009, equivalent to a sea level rise of 0.13 and 0.22 mm yr− 1, respectively. The acceleration is due to a combination of decreased snowfall accumulation (+ 13 Gt yr− 1 in 2002–2006, and − 6 Gt yr− 1 in 2007–2009) and enhanced ice dynamic thinning (− 60 ± 10 Gt yr− 1 in 2002–2006, and − 74 ± 11 Gt yr− 1 in 2007–2009) after 2007. Because there is no significant snowfall trend over the past 21 yr (1989–2009) and an increase in ice flow speed (2003–2010), the accelerated mass loss is likely to continue."

The caption for the attached image is:
"Spatial distributions of the standard deviations of the Envisat time series from September 2002 to December 2009 for all 1° × 1° regions (a) before and (b) after the surface gradient correction using the Antarctic DEM. The AS sector is shown with dashed lines. Spatial plots of elevation change rates (c) and their formal uncertainties (d) are also shown"


For a more current discussion of the GIA for the Amundsen Sea sector please go to:

http://www.sciencedirect.com/science/article/pii/S0921818112001567
« Last Edit: June 29, 2013, 04:58:22 PM by AbruptSLR »
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #39 on: July 08, 2013, 05:21:46 PM »
I thought that I would post the two attached images from a AGU presentation by Alley June 2013.  The first image shows how the current ice flow lines passing through the Thwaites Gateway crowd together to restrict ice mass loss through the limited with of the current gateway; while if the grounding line retreats by about 100km upstream (Southward) then the crowding effect would become much less, so the ice mass loss is expected to accelerate due to geometrical considerations alone.  The second attached image reinforces the same geometrical message as the first image except in terms of horizontal ice velocities through the Thwaites Gateway.
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #40 on: July 10, 2013, 07:16:29 AM »
This linked article seems to be new (to me anyway!) information about Thwaites Glacier and information on how the water underneath Glaciers affects their movement.
The University of Texas at Austin’s Institute for Geophysics have used an innovation in radar analysis to accurately image the vast subglacial water system under West Antarctica’s Thwaites Glacier. They have detected a swamp-like canal system beneath the ice that is several times as large as Florida’s Everglades.

Distinguishing subglacial swamps from streams is important because of their contrasting effect on the movement of glacial ice. Swamp-like formations tend to lubricate the ice above them whereas streams, which conduct water more efficiently, are likely to cause the base of the ice to stick between the streams.
  http://www.utexas.edu/news/2013/07/09/scientists-image-vast-subglacial-water-system-underpinning-west-antarctica’s-thwaites-glacier/

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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #41 on: July 10, 2013, 03:07:57 PM »
JMP,

Thanks, the linked information provides expanded (new) information on the work presented by Dustin Schroeder et al that I discuss in the Surge thread, replies #1, 72, and 73 at:

http://forum.arctic-sea-ice.net/index.php/topic,21.0.html

And of course the information in the Subglacial Lakes & Meltwater Drainage Systems thread is also relevant at:

http://forum.arctic-sea-ice.net/index.php/topic,404.0.html

The concept of a swamp (rather than a subglacial lake as I had previous assumed) upstream from the Thwaites Gateway is an important refinement on earlier work.

Thank you,
ASLR
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #42 on: July 10, 2013, 07:35:53 PM »
For those who are interested in the topic that JMP posted on, the paper entitled: Evidence for a water system transition beneath Thwaites Glacier, West Antarctica; by: Dustin M. Schroeder, Donald D. Blankenship, and Duncan A. Young; July 2013; can be downloaded at the following link:

http://www.pnas.org/content/early/2013/07/03/1302828110.full.pdf+html
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #43 on: July 11, 2013, 12:42:09 AM »
I realize that I have posted about the Pine Island and Thwaites Glaciers in many different threads, and recently on to significant papers in replies #78 and 88 of the "Collapse" thread here:

http://forum.arctic-sea-ice.net/index.php?topic=31.50

Nevertheless, this is the most appropriate thread for posting the attached images that come from the following PowerPoint:

Recent Changes in Greenland & Antarctica
April 23, 2013 @ Ice and Climate, ATM S 514; by: Kristin Poinar and Ian Joughin, from the Polar Science Center

The first image (after Jacobs 2011) shows the increase in ice melting potential temperature difference (as a future of depth and year) for the warm CDW entering the Pine Island Ice Shelf from 1994 until 2009.  As I have shown evidence that this sam CDW continues on to the Thwaites Trough; where it represents a very serious threat to activate the Thwaites Glacier from now until 2060.

The second image (after Alley 2012) discusses how high tides flex up ice shelves and ice tongues such as the Thwaites Ice tongue, thus forming a basal gap allowing seawater to flow some distance (on the order of a kilometer) up the gateway of the glacier; while low tides flex down the ice shelves and ice tongues, thus squeezing out the seawater that was introduced during the previous high tide.  This action degrades the gateway ice.

The third image (from Joughin) shows how the outer face ice velocity for the Jakobshavn Glacier accelerated from about 4,000 m/yr in 1995 to about 8,000 m/yr in 2003 to about 16,000 m/yr in 2012 primarily due a combination of warming local ocean water temperatures and a loss of buttress support.  I have repeatedly stated that I foresee such a trend beginning for the Thwaites Glacier possibly within about 10 years.

The fourth image shows a floating ice melange infront of the Jakobshavn Glacier in 2004 following the 2003 acceleration event.  I have repeatedly that a acceleration of the Thwaites Glacier could lead to such a melange in the Thwaites Gateway before 2060.
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #44 on: July 11, 2013, 05:38:41 PM »
I believe that my comments about the second attached image in my immediately proceeding post (reply #43) contained errors; as I now believe that Richard Alley was most probably talking about the tidal influence on basal gaps (and intruding seawater) beneath the Siple Coast Ice Streams and most likely not the Thwaites Ice Tongue.  This is important to correct for reasons including the following:

- The offshore end of the Thwaites Ice Tongue is pinned by a submerged ridge; which stops the outer end of the tongue from flexing up and down with the tides.  I believe that this submerged ridge keeps the outer end of the tongue generally flexed up (in cantilever action), thus contributing to the existence of a basal gap (at the downstream portion) beneath the ice stream in the Thwaites Trough; which tidal water can then flow into, and out of (together with the fresh upstream meltwater flowing out through the trough); which in my opinion accelerates the formation of a new subglacial cavity in the Thwaites Trough.
- I have noted elsewhere (see the "Surge" thread) that the Thwaites Ice Tongue not only surged in the Fall of 2012 but also in 2002; leading me to speculate that in the ten year period the subglacial cavity in the Thwaites Trough grew to be long enough that stresses associated with tidally induce simple beam flexural stresses (with a beam span length from the offshore submerged ridge to the grounding line of the subglacial cavity in the Thwaites Trough by September 2012), caused the old ice tongue to crack vertically near mid-span, causing the old tongue to be rotated out of the way as the new tongue surged outward in the Fall of 2012 (see the "Surge" thread).
- I also believe that as the volume and temperature of the warm CDW currently forming a new subglacial cavity (note that the old cavity appears to have infilled when the tongue surged in 2012) in the Thwaites Trough; the time duration until the next tongue surge event will be less than ten years (particularly considering that the end of the current El Nino hiatus period will soon episodically drive more warm CDW into the ASE); which, will sufficient cycles could thin the overlaying ice sufficiently to lead to the formation of a permanently floating ice shelf in the Thwaites Trough, resulting in a direct channel for warm CDW to spread out laterally near the lip leading down into the Byrd Subglacial Basin, BSB (see the maps attached to the first post in this thread).
- The lateral spreading of warm CDW along the lip leading down into the BSB, could within several decades float much of the ice in the current Thwaites Gateway; which could inturn lead to a collapse mechanism for the Thwaites Glacier (see the low basal friction area in the "swampy" area, upstream of the Thwaites Gateway, shown in the attached image from Schroeder et al 2013) which could trigger a Jakobshavn (or Thwaites) Effect, as I have previously discussed in this (and other) threads.
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #45 on: July 20, 2013, 03:53:07 PM »
I have previously talked at length about the risks that the end of the current El Nino hiatus period presents particularly with accelerating ice mass loss from the Thwaites Glacier without adequately discussing some aspects of my assumptions in the 2012 to 2040 timeframe:

I have assumed that the hiatus period will end between 2014 and 2017, and that the associated influx of warm surface seawater will cause the Thwaites Ice Tongue and Ice Shelf to largely collapse by 2025; which will both: (a) contribute to rapid thinning of the ice stream in the Thwaites Trough, with an associated acceleration of the subglacial cavity formation in the trough so that by about 2040 the grounding line, GL, in the trough has retreated back to about the location of the submerged mount in the Thwaites Gateway; and (b) eastward of the Thwaites Trough the GL will retreat in an accelerated fashion partially due to the turbulent convection process illustrated in the attached image that shows that the combination of warm ocean water and fresh basal meltwater emerging from beneath the glacier can cause accelerated mixing due to bouyancy effects.

I assume that the positive PDO (Pacific Decadal Oscillation) period (signalling the end of the hiatus period) after about 2014 to 2017 will last for about 20 to 30 years; by which time I assume that the GL will have retreated back to the subglacial lake infront of the submerged mount in the Thwaites Gateway; and thereafter the retreat of the GL down the negative slope into the BSB will accelerate without the assistance of periodic El Nino events (associated with the positive PDO); shortly thereafter leading to the "Thwaites Effect" that I have previously postulated.
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #46 on: July 24, 2013, 08:17:45 PM »
While I made a less complete post on this topic in the "Collapse" thread; the information contained in the following University of Michigan website and in the following referenced paper, abstract, and attached figure; is of such fundamental importance to the "Thwaites Effect" discussed in this thread, that I am making this additional post on this topic:

http://www.ns.umich.edu/new/multimedia/videos/21600-sea-level-rise-new-iceberg-theory-points-to-areas-at-risk-of-rapid-disintegration

Diverse calving patterns linked to glacier geometry
by:J. N. Bassis & S. Jacobs; Nature Geoscience; (2013); doi:10.1038/ngeo1887

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."

This paper (and accompanying find) indicate that the actions of the submerged seamount in Thwaites Gateway to buttress the thicker upstream ice; and of the high snowfall rates in this area to thicken the upstream ice; actual contribute to the instability of the Thwaites Glacier once the: (a) calving; and (b) advective basal ice melting; have cleared the ice downstream and around the submerged seamount in the Thwaites Gateway.
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #47 on: July 25, 2013, 06:45:16 PM »
I seem to have commented on the Bassis paper on a different thread. Sorry!

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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #48 on: August 18, 2013, 05:09:36 PM »
The following reference (the link provides a free pdf) and associated Conclusions (which I like more than the abstract), emphasizes the importance of kilometer scale variations in ice shelf melting.  This work indicates that averaging these kilometer scale variations results in non-conservative (w.r.t. public safety) results regarding the risk of accelerating SLR:

http://www.the-cryosphere-discuss.net/7/1591/2013/tcd-7-1591-2013.pdf

Pine Island Glacier ice shelf melt distributed at kilometre scales
by: P. Dutrieux, D. G. Vaughan, H. F. J. Corr, A. Jenkins, P. R. Holland, I. Joughin, and A. Fleming; The Cryosphere Discuss., 7, 1591–1620, 2013; www.the-cryosphere-discuss.net/7/1591/2013/; doi:10.5194/tcd-7-1591-2013

Conclusions:
"Previous work has indicated high melt-rates near the grounding line of PIG (Payne et al., 2007; Rignot and Jacobs, 2002) and the presence of basal channels in the ice (Bindschadler et al., 2011; Mankoff et al., 2012; Vaughan et al., 2012). Our observations show that the pattern of melting on PIG ice shelf is highly complex. Within 10km of the grounding line, the melt rate is at least 100myr−1. Only 20 km downstream this reduces to 30myr−1. Between 2008 and 2011, basal melting was largely compensated by ice advection, allowing us to estimate an average loss of ice to the ocean of 87 km3 yr−1, in close agreement with 2009 oceanographically-constrained estimates.  Close to the grounding line, melting is concentrated in the basal channels and carves out those channels at 80myr−1. Further downstream, melting on the keels is 30myr−1 faster than in the channels, which explains the gradual loss of channels in the downstream part of the ice shelf and the inversion of the surface elevation anomalies relative to free floatation. The gradual regime shift in channel melt could be explained by the initial formation, near the grounding line, of buoyant meltwater plumes rising up the ice base and most efficiently entraining heat to the channel crests, and a decrease in the heat entrainment efficiency downstream as the slope weakens, the ice base shallows and the warm water source gets further away. At some stage, the plumes within the channels deliver less heat to the ice shelf than the warmer deeper waters bathing the channel keels.  With the advent of ice surface DEMs of even higher resolution (few meters) taken at regular time intervals, we can expect that the methodology developed here will reveal unforeseen details about the distribution of surface elevation changes and by inference of basal melt where the underlying assumptions are valid, thereby increasing our understanding of atmosphere-ice-ocean interaction dynamics and their temporal and spatial variability.

Our observations of the area close to the grounding line therefore indicate melt rates that are 80% higher in channels than on neighbouring keels, and point to high spatial variability in the melt-rates across the ice shelf, indicating strong modulation of ice-ocean interactions at kilometre scales. This implies that in-situ observations need to be interpreted within their contextual position relative to the channels. Possibly the most important implication of this work concerns the modelling of sub-ice shelf cavities. Accurately representing sub-kilometre scales using conventional ocean models is challenging even for dedicated regional studies, and will remain impossible for global coupled climate models for some time to come. One approach to solving this problem is to use unstructured computational meshes to focus the model resolution on features of interest, such as these channels (Kimura et al., 2013; Timmermann et al., 2012).  A more conventional alternative would be to parameterise their effect on the larger scales that models are able to resolve. For either of these approaches to be successful, an essential prerequisite is a detailed observational understanding of the channels, for which the present study provides a significant advance."
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Re: Hazard Analysis for PIG/Thwaites from 2012 to 2040-2060 Timeframe
« Reply #49 on: August 23, 2013, 02:30:23 AM »
I posted this ENSO figure (and associated weblink) in the "Antarctic Weather and Meteorology" thread; but as I am concerned that some people may miss it over there, I am re-posting this image here as the possible return of an El Nino event in the mid-2014 could have serious consequences on ice mass loss from the Thwaites Glacier.
“It is not the strongest or the most intelligent who will survive but those who can best manage change.”
― Leon C. Megginson