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: NSF grants , , , , CReSIS NASA grant NRA-04-OES-02 Gary Comer Science and Education Foundation

Jan. 30, 2002 Scambos, NSIDC 30 km

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

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

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

mages/content/95798main_larsen_zoo m2_m.jpg Antarctica: meltwater wedges open crevasses and destroys shelves. Greenland: ice arrives at shelf with plumbing, so meltwater doesn’t destroy shelves. Warm Greenland shelves do not provide guidance on Antarctic. Warming is different than warm!

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

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 This Study:

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 (1 o C 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 (1 o C oceanic warming  10 m/yr basal melting; Rignot and Jacobs, 2002)

The Isothermal Flowline Model Conservation of momentum: Conservation of mass:

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) T=20 kyrs; t: [0,50kyrs]; t ss ~ 2652 yrs Le: [300,3000] m

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.

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

Jakobshavn Glacier Flow Image courtesy of I. Joughin 15 km

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

Ice-Front BC T n if = 0 above the water line T n if =  sw gzn x ; 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