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Effect of Climate Cycling and Meltwater Plumbing on Ice-Sheet Grounding-Line Migration

Effect of Climate Cycling and Meltwater Plumbing on Ice-Sheet Grounding-Line Migration. Byron R. Parizek 1 , Richard B. Alley 1 , Todd K. Dupont 2 , Ryan T. Walker 1 , and Sridhar Anandakrishnan 1. 1 The Pennsylvania State University 2 University of California, Irvine. Funding provided by:

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Effect of Climate Cycling and Meltwater Plumbing on Ice-Sheet Grounding-Line Migration

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  1. Effect of Climate Cycling and Meltwater Plumbing on Ice-Sheet Grounding-Line Migration Byron R. Parizek1, Richard B. Alley1, Todd K. Dupont2, Ryan T. Walker1, and Sridhar Anandakrishnan1 1The Pennsylvania State University 2University of California, Irvine Funding provided by: NSF grants 0531211, 0758274, 0809106, 0814241, CReSIS NASA grant NRA-04-OES-02 Gary Comer Science and Education Foundation

  2. Jan. 30, 2002 Scambos, NSIDC 30 km

  3. Mar. 04, 2002 Scambos, NSIDC 4-8 fold Increase In velocity 30 km

  4. Ice-shelf buttressing of outlet-glacier flow matters! (over glacial-interglacial cycles too… DeConto and Pollard)

  5. A Paradox? • Breakup of Larsen B vs Ice Shelves in Greenland • Followed atypical warmth of • 2001-02 melt season: • mean monthly T ~4oC • meltwater production up • 3-fold to ~40 cm/yr • For at least past 50 yrs, they have been exposed to: • summer mean T ~ 3-11oC • melt rates that can exceed 250 cm/yr

  6. Antarctica: meltwater wedges open crevasses and destroys shelves. Greenland: ice arrives at shelf with plumbing, so meltwater doesn’t destroy shelves. http://www.nasa.gov/centers/goddard/images/content/95798main_larsen_zoom2_m.jpg Warm Greenland shelves do not provide guidance on Antarctic. Warming is different than warm!

  7. Hypothesis: The advection of englacial meltwater channels that develop upglacier can drain surface meltwater, thereby limiting ponding within crevasses and subsequent shelf failure. (Zwally et al., 2002; Das et al., 2008)

  8. This Study: • Isolate the effect of climate cycling and meltwater plumbing on ice shelves (and, through buttressing variability, on • outlet-glacier flow and grounding-line motion) •  Ice-shelf perturbation experiments using reduced-dimensional modeling of an idealized outlet system

  9. Removing the ice: • Heat from Earth melts few millimeters of ice per year, ice-flow friction heat a bit more (but some heat may be conducted into ice so not melting); • Heat from air melts few meters of ice per year in ablation zones (1oC atm warming1/3-2/3 m/yr melt during summer season; Braithwaite, 1995) • Heat from ocean can melt 10s meters of ice per year (1oC oceanic warming 10 m/yr basal melting; Rignot and Jacobs, 2002)

  10. The Isothermal Flowline Model • Conservation of momentum: • Conservation of mass:

  11. T=20 kyrs; t: [0,50kyrs]; tss~ 2652 yrs Le: [300,3000] m • 15 simulations: • Standard: no calving threshold, • Bdot = ave(Bdot), Adot = 0 • ``L’’-type calving: 12 m/yr (10) • ``LGL”-type: 12, 17, 12 m/yr (10,15) • ``LG”-type: 12, 17 m/yr (10,15) • Bdot(x), Adot=0 • Melt partioning: 83% Bdot, 17% Adot • (<=25 m/yr, <=5 m/yr)

  12. Conclusions: • hysteresis loop without calving thresholds arises from ice-shelf buttressing interactions with outlet glacier in an oscillating climate • (rapid loss and delayed establishment of ice-shelf buttressing) • basal-melt distribution enhances the hysteretic behavior • With calving thresholds, the hysteresis loops widen • Implications for the future of the Larsen ice shelves (will plumbing be established with readvance possible or will the ocean warm faster) • Differs in forcing and character from thermomechanical binge-purge behavior. Taken together, these processes would likely enhance the hysteretic response of coupled sheet-stream-shelf systems.

  13. Pine Island and Thwaites Glaciers (Rignot et al., 2004)

  14. Jakobshavn Glacier Flow 15 km Image courtesy of I. Joughin

  15. Speedup of Grounded Ice After Ice-Shelf Collapse 1992 2000 (Joughin et al., Nature, 2004)

  16. Ice-Front BC • T nif = 0 above the water line • T nif = rswgznx; hydrostatic beneath water line • pressure imbalance --> stretching required to balance stresses • Buttressing reduces this stretching tendency by drag at the sides or on ice rises

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