Reposting 2017: What is Antarctica’s Future?

December 30, 2017

In advance of a giant break in the Larsen C ice shelf in Antarctica, I interviewed glaciologists Eric Rignot of NASA, and Jeremy Bassis, of the University of Michigan.

7 Responses to “Reposting 2017: What is Antarctica’s Future?”

  1. grindupbaker Says:

    Based on the fracture of the Larsen C ice shelf first being noticed in November 2010, its extent and width at that time and its rate of growth the few years following, I suggest the possibility that the fracture of the Larsen C ice shelf might well have been caused by the Indian Ocean tsunami 26 December 2004 following sea bed earthquake. “The tsunami also reached Antarctica, where tidal gauges at Japan’s Showa Base recorded oscillations of up to a metre with disturbances lasting a couple of days”.

    I see “the sea floor is estimated to have risen by several metres, displacing an estimated 30 of water”. If I take the tsunami as radiating in a circle then the radius is 13,000 km at Larsen C ice shelf distance so the quantity of tsunami water per metre of impacted face is 30,000,000,000 / (26,000,000 * pi) = 367 m**3 (this assumes negligible settling of the water during travel). For 1 metre of SLR extending to 367m from the ice shelf face I compute 367 * 42,000 * 10,000 = 154,000,000,000 newton-metres of torque per metre of fracture run at the fracture point using a 42km width. If I assume 350m thick then the tensile pull at the bottom of the fracture from 1m SLR lifting at 42 km from the pivot point = 440,000,000 newtons per metre of fracture run. The tensile pull over 350m thick from 1m SLR lifting over 42 km = 1,260,000 newtons per metre of ice depth per metre of fracture run (i.e. per square metre) average throughout ice depth. However, (595-435)/595=27% so the lowest 50m of the ice shelf face is subjected to 27% of the torque force, so tensile pull over the lowest 50m of the ice shelf face = 2,380,000 newtons per metre of ice depth per metre of fracture run (i.e. per square metre). The tensile strength of ice varies from 0.7–3.1 MPa so the fracturing force exerted on the ice shelf at the fracture location from 1 metre of SLR would be anywhere between 0.8x and 3.4x that required to fracture it (if ice were infinitely brittle) so it is definitely of the order of magnitude to be very possible based on the 367 m**3 simultaneously per metre of impacted face.

    Of course, ice has some ductility & malleability (not perfectly brittle) and tides there are of order 1m to 1.7m, same as that tsunami or somewhat higher, so the ice shelf could not survive tides if it was perfectly brittle. Davis tide table indicates typically 14 hours for the tide to rise 1m to 1.7m but likely the far more rapid impact force of a tsunami SLR (over a few minutes I assume) would not give the ice shelf sufficient time to respond elastically throughout its length and it fractured along its weakest line on the lower face due to the torque exerted. This would open a fracture 7 mm wide at 42 km back from the face if the ice did not yield anywhere except at the fracture so, for example, if the ice bent 90% of the required amount to relieve stress throughout its length then it would open a fracture 0.7 mm wide. Would need structural analysis to figure it out properly.

    The line from the centre of the tsunami origin to the centre of the Larsen C ice shelf is at an angle close to perpendicular so SLR would have been applied across a large width of the face simultaneously. Extrapolating back in time from the fracture distance increase between 2010-11 and 2015-10 indicates a fracture date of 2002-05 which is 2.5 years before the tsunami so it doesn’t support the December 2004 date strongly but given the uncertainty in that method it doesn’t rule it out (perhaps there was some initial length of fracture before it started increasing).

    The only significant contraindication is that it appears that a straight line across the ocean from the centre of the tsunami origin to the centre of the Larsen C ice shelf is interrupted by the western edge of Queen Maude Land, in which case there would be no direct wave front across all of the centre of the Larsen C ice shelf but only the portion of the original wave that spreads southwards. I’m not sure because I don’t own an Earth globe and my friend kept telling me to turn her Earth globe upright again and put it back on the ornaments table.

    • Torsten Says:

      You’d also have to have some knowledge of the local bathymetry because it affects the height of the tsunami as well as the possibility of resonance effects. It’s a straight, uninterrupted line from the center of that earthquake to parts of the coasts of Argentina and Uruguay. The amplitude of the tsunami varied from 0.15 m at one station to more than 1.2 m at another along that coastline. On the other hand, it’s closer and also on a straight line from the earthquake center to Dumont D’Urville, Antarctica, where the tsunami was 0.06 m.

      I’m not sure how much the sampling interval affects the determination of maximum wave height. It varied from 4-6 minutes for three of the South American stations, to 30 minutes for the Antarctic one I mentioned.

      • grindupbaker Says:

        Height of the tsunami alone is irrelevant to the fracture capability. It affects only the fracture width (if the force manages to fracture). As far as quantity & height go, it’s my first estimate of 367 m**3 simultaneously per metre of impacted face that decides whether it fractures. As stated, the available information is minimal so it’s a highly-tentative quantity, but not unreasonable. I forgot to include that Syowa Japan base Antarctica measured up to 1m tsunami height, but, again, the height of the tsunami is irrelevant to the fracture capability, it’s the force & speed that are relevant. So, the measured heights would, indeed, be useful if, and only if, the front-rear length of the tsunami wave was also known at that place so that the quantity, which is what causes the lifting and lowering forces, could be computed. Is the tsunami wave velocity for the 30 minutes for the Dumont D’Urville, Antarctic one at 0.06 m known ?

        • grindupbaker Says:

          This is Web-Log-quality assumed and understood limitations of course. A tsunami wave 3,670 km long and 0.1 mm tall will not fracture an ice sheet because speed definitely too slow. It’s within reasonable practical ranges.

  2. indy222 Says:

    Expecting the ice shelves and continental ice to melt like an ice cube would, on a table at room temperature (as I believe the older IPCC models did) seems even less defensible when you consider that the thickness of these ice sheets may be “thick” in human terms; a 1000 ft or even 10,000 ft thick, but their form factor is not unlike a sheet of paper when you realize how many miles across they are. breaking and fracturing and all the rest, for a warming near 0C “sheet of paper” of ice, is pretty easy to believe.

    • grindupbaker Says:

      “not unlike a sheet of paper….for a warming near 0C “sheet of paper” of ice, is pretty easy to believe”. Yes, as is fracture from a rapid lifting or lowering force applied to a thin sheet.

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